Here is the Prime Minister's letter in full.
Thank you for your correspondence of 19 December 2019.
I have carefully considered and noted the arguments set out for a transfer of power from the UK Parliament to the Scottish Parliament to allow for further independence referendums.
You and your predecessor made a personal promise that the 2014 Independence Referendum was a "once in a generation" vote. The people of Scotland voted decisively on that promise to keep our United Kingdom together, a result which both the Scottish and UK Governments committed to respect in the Edinburgh Agreement.The UK Government will continue to uphold the democratic decision of the Scottish people and the promise that you made to them. For that reason, I cannot agree to any request for a trasnfer of power that would lead to further independence referendums. Another independence referendum would continue the political stagnation that Scotland has seen for the last decade, with Scottish schools, hospitals and jobs again left behind because of a campaign to separate the UK.
It is time that we all worked to bring the whole of the United Kingdom together and unleash the potential of this great country.